This invention relates generally to mechanical transmission apparatus and, more particularly, to gear-based transmission apparatus having reduced backlash effects and little or no cogging effects.
Gear based transmission represents a very rigid and cost effective method for transmitting large forces at a very low cost when compared to cable and belt based transmissions. Gear based transmissions, however, must maintain clearance between the mating teeth of the gear sets that make up the gear train. This clearance, called backlash, exists in even the most precise of mechanisms to allow for misalignment, manufacturing variances, and material expansion and contraction due to temperature changes. In devices that are designed to transmit a torque or other forces in rapidly changing environments, such as force feed back mechanisms in simulator input devices, this backlash diminishes the response of the transmission system. For instance, when the input device needs to reverse the transmission of a force, the backlash causes a delay in the system response. This delay is particularly problematic for devices that use gear based transmissions to transmit torque and reverse direction rapidly while maintaining a high frequency response.
As the frequency of the system increases, the forces transmitted by the input device may decrease to such a degree that the time required to fully reverse the transmission exceeds the time needed to remove the clearance between the teeth, thereby resulting in no transmission of force at all. The problem of backlash limits the ability of the device to simulate effectively actual real world behavior, reducing its value in program simulation.
Moreover, the motors used in the apparatus typically have an inherent detent position, which causes a stepping action known as the cogging effect that is caused by the interaction of magnets and poles of the rotor. Due to the cogging effect, the output shaft of the device exhibits an undesirable notched or stepped feeling during use.
Embodiments of the present invention are directed to transmission apparatus that are configured to largely eliminate the cogging effects and to reduce the backlash effects. Controlling the backlash effects and reducing or eliminating cogging effects are particularly advantageous, for instance, for devices that use gear based transmissions to transmit torque and reverse direction rapidly while maintaining high frequency response. In specific embodiments, a dual motor, single reduction transmission is used to eliminate the cogging effects and reduce the backlash effects, and to improve the frequency response of the transmission. The dual motor transmission includes two motors with motor pinions for driving a follower gear, preferably via a single gear reduction. By controlling the motors to rotate out of phase with respect to one another, the cogging effects are reduced or eliminated. The highs and lows of the detent torques of the two motors are smoothed out when combined. As one motor approaches its maximum resistance to rotation and thus its minimum rate of change, the other motor is half of a cycle behind the first motor at its minimum resistance to rotation and thus its maximum rate of change. When the motors continue to rotate, the decrease in resistance of one motor is matched with the increase in resistance of the other motor.
In addition, the use of two motors coupled with a single reduction gear allows the transmission to achieve the same output torque as a single motor, double reduction transmission, by providing twice the motor torque to offset reducing the gear reduction by half. By eliminating the second reduction, however, the total backlash is reduced by more than about 50%. This reduction in backlash results from the reduced complications involved with tolerancing two sets of gears in a double reduction transmission.
In accordance with an aspect of the present invention, a transmission apparatus comprises a follower gear, a first drive gear set coupled with the follower gear to drive the follower gear, and a second drive gear set coupled with the follower gear to drive the follower gear. The first drive gear set includes at least one gear, and the second drive gear set includes at least one gear. A first motor is coupled with the first drive gear set. The first motor includes a first rotor having a plurality of poles which are movably disposed relative to a plurality of magnets. A second motor is coupled with the second drive gear set. The second motor includes a second rotor having a plurality of poles which are movably disposed relative to a plurality of magnets. The disposition of the poles relative to the magnets of the first motor is out of phase with respect to the disposition of the poles relative to the magnets of the second motor.
In some embodiments, only one of the poles is aligned with one of the magnets in an alignment position for the first motor at any time, and only one of the poles is aligned with one of the magnets in an alignment position for the second motor at any time. The alignment position for the first motor and the alignment position for the second motor do not occur simultaneously. The first motor reaches the alignment position periodically over time with a first time period, and the second motor reaches the alignment position periodically over time with a second time period. The alignment position of the first motor falls substantially midway in time between two adjacent alignment positions of the second motor. The first time period and the second time period are equal.
In specific embodiments, the first motor comprises three poles angularly spaced around the first rotor, and the second motor comprises three poles angularly spaced around the second rotor. The first drive gear set consists of a first pinion and the second drive gear set consists of a second pinion.
In accordance with another aspect of the invention, a transmission apparatus comprises a follower gear, a first drive gear set coupled with the follower gear to drive the follower gear, and a second drive gear set coupled with the follower gear to drive the follower gear. The first drive gear set including at least one gear, and the second drive gear set including at least one gear. A first motor is coupled with the first drive gear set. The first motor includes a plurality of poles which are movably disposed relative to a plurality of magnets to produce a torque which varies in time between a first maximum level and a first minimum level. A second motor is coupled with the second drive gear set. The second motor includes a plurality of poles which are movably disposed relative to a plurality of magnets to produce a torque which varies in time between a second maximum level and a second minimum level. The torque of the first motor does not reach the first maximum level at the same time that the torque of the second motor reaches the second maximum level. The torque of the first motor does not reach the first minimum level at the same time that the torque of the second motor reaches the second minimum level.
In some embodiments, the torque of the first motor reaches the first maximum level at about the same time the torque of the second motor reaches the second minimum level. The torque of the first motor reaches the first minimum level at about the same time the torque of the second motor reaches the second maximum level. The first maximum level of the torque of the first motor is substantially the same as the second maximum level of the torque of the second motor. The first minimum level of the torque of the first motor is substantially the same as the second minimum level of the torque of the second motor.
In accordance with another aspect of this invention, a method of reducing or eliminating cogging effects in a transmission device comprises providing a first drive gear set which includes at least one gear, providing a second drive gear set which includes at least one gear, and coupling the first drive gear set and the second drive gear set with a follower gear. A first motor is coupled with the first drive gear set, and a second motor is coupled with the second drive gear set. The first motor includes a first rotor having a plurality of poles movably disposed relative to a plurality of magnets. The second motor includes a second rotor having a plurality of poles movably disposed relative to a plurality of magnets. The first motor and the second motor are positioned to maintain a disposition of the poles relative to the magnets of the first motor to be out of phase with respect to a disposition of the poles relative to the magnets of the second motor.
In accordance with another aspect of the present invention, a method of reducing or eliminating cogging effects in a transmission device comprises providing a first drive gear set which includes at least one gear, providing a second drive gear set which includes at least one gear, and coupling the first drive gear set and the second drive gear set with a follower gear. A first motor is coupled with the first drive gear set, and a second motor is coupled with the second drive gear set. The first motor includes a first rotor having a plurality of poles movably disposed relative to a plurality of magnets. The second motor includes a second rotor having a plurality of poles movably disposed relative to a plurality of magnets. The first rotor of the first motor and the second rotor of the second motor are positioned to be out-of-phase with respect to one another such that the torque of the first motor and the torque of the second motor do not reach the maximum level at the same time and do not reach the minimum level at the same time.